1. Field of the Invention
The present invention relates to the improvement of alkaline storage batteries which use nickel hydroxide for the positive electrode.
2. Description of the Related Art
Alkaline storage batteries widely used for electric sources of various portable apparatuses include nickel-cadmium storage batteries, nickel-metal hydride storage batteries and the like.
Hitherto, many inventions and proposals have been made on nickel hydroxide which is commonly used for the positive electrode of these alkaline storage batteries and plays an important role, and improvements have been made from various view points (for example, improvement of high-temperature charge efficiency, improvement of utilization factor and prolongation of cycle life). Specifically, mention may be made of addition of conductive agent powder (especially, Co, Co compounds, Ni, carbon and the like) for effective utilization of charge-discharge characteristics of nickel hydroxide powder and design of substrates for current collection. Furthermore, various proposals have been made on improvement of nickel hydroxide per se.
With regard to the improvement of nickel hydroxide per se, as a relatively old proposal, Mg, Mn, Zn, Pb, Ba, Fe, rare earth elements are dissolved in nickel hydroxide to form a solid solution as reported in "Power Source" (1966), pp239-255. However, no advantage as a positive electrode active material for alkaline storage batteries is mentioned there on the dissolution of Mn as solid solution. JP-A-51-122737 proposes a process of precipitating a hydroxide by reacting a mixed aqueous solution of nickel and a salt of metal other than nickel with an aqueous alkaline solution as a measure to simplify the steps and reduce the production cost. As relatively new proposals, JP-A-61-101958 proposes dissolution of Co as solid solution and JP-A-61-104565 proposes dissolution of Cd as solid solution for the improvement of utilization factor of active materials in high-temperature atmosphere. Moreover, JP-A-4-179056 proposes to dissolve Mn or the like in nickel hydroxide to form a solid solution as a method for improving cycle life characteristics. In this method, charge-discharge reaction of active material is basically a one-electron reaction between .beta.-Ni(OH).sub.2 and .beta.-NiOOH. Therefore, these techniques do not directly contribute to the improvement of capacity density of batteries. In addition, JP-A-8-222215 and JP-A-8-222216 make proposals on nickel hydroxide in which divalent or trivalent Mn is dissolved to form a solid solution. However, these proposals cannot still provide a utilization factor of active materials which is far higher than 110% (1.1 in terms of the number of electrons participating in reaction for Ni).
On the other hand, JP-A-8-225328 proposes to dissolve trivalent Mn in nickel hydroxide to form solid solution, thereby making it possible to perform charging and discharging of more than one-electron reaction. U.S. Pat. No. 5,348,822 also proposes a technique to make it possible to perform charging and discharging of more than one-electron reaction. According to these proposals, charging and discharging of more than one-electron reaction become possible by stabilizing the reaction between .alpha.-Ni(OH).sub.2 and .gamma.-NiOOH. The .alpha.-Ni(OH).sub.2 used there is very low in its density, and improvement of capacity density (capacity per unit volume) of electrode is very difficult. Thus, these proposals are considered to be low in practical value.
In the above conventional nickel hydroxide (positive electrode active material for alkaline storage batteries), there is yet room for improvement on capacity density, utilization factor and others. That is, in conventional active materials, the number of electrons participating in reaction for nickel is basically 1 to 1.1, and even those which perform the charging and discharging reaction exceeding the above number of electrons cannot be improved in capacity density (capacity per unit volume) because of low density of active materials per se. Thus, conventional active materials have not sufficiently contributed to increase the capacity of batteries, which is always demanded by the market for using them as electric sources of portable apparatuses.